1. Field of the Invention
The present invention relates to a field effect transistor (FET) which operates with low noise at a high speed and can be fabricated with high uniformity, and a method for manufacturing the same.
2. Related Background Art
An FET which has a GaInAs/GaAs quantum well structure and has a GaInAs layer as a channel is disclosed in JP-A-63-90861, JP-A-63-272080 and JP-A-64-2371. Those FETS utilize the fact that GaInAs has a higher electron mobility than GaAs and overcomes a drawback that GaInAs cannot form a good Schottky barrier with the ability of GaAs by replacing GaInAs with GaAs to attain a high speed operation. However, demands for high speed are still strong and it has been proposed to form a low resistivity area by ion implantation to the source and drain regions of an FET having a GaInAs/GaAs quantum well structure to reduce stray serial resistance in order to meet those demands.
In such an FET, however, there is a risk of breakdown for the quantum well structure in an annealing process of the low resistivity area due to a difference between lattice constants of GaInAs and GaAs.
IEEE Electron Device Letter, Vol. 9 1988, pp 621-623 and "A study of the thermal stability of (In, Ga)As strained quantum well structures as a function of overlayer thickness" of Inst. Phys. Conf. Ser. No.96 chapter 6, page 365-370, which is presented at Symp. GaAs and Related Compounds, Atlanta, Ga. 1988, teach that the thermal stability of the GaInAs/GaAs quantum well structure is significantly improved by increasing the thickness of a GaAs cap layer on GaInAs (to 500 nm). However, when the GaAs cap layer on GaInAs is thickened and a gate electrode is formed on the GaAs cap layer, current in an n-type GaInAs channel cannot be fully modulated and good characteristics for the FET cannot be attained. In order to solve this problem, it has been proposed to form a groove called a recess when a gate electrode is to be formed, and form the gate electrode at the bottom of the recess in order to reduce a distance between the channel and the gate electrode, but it is technically difficult to form a recess having a uniform depth and therefore uniformity of the characteristic of the FET is lost.